The assessment and control of moisture content of natural or synthetic materials is considered important in several industries such as pharmaceuticals, food, agrochemicals, plastics, construction, mining, paper manufacture, catalyst manufacture, petrochemicals, semiconductors, etc. In several cases, the moisture content is taken as an indication of the quality of the product.
It is recognized that practically all manufacturing processes require strict control of the fluid levels therein, at times at ppm levels, in order to comply with quality requirements, regulatory standards, prevent defects in the final product of such processes, etc. For example, the presence of unwanted moisture (water) in granule manufacture in pharmaceuticals can have an adverse effect on the product in terms of agglomeration of granules, leading to rejection of product batches. Another example involves the treatment of polymeric materials. During the processing of various polymeric materials, it is imperative to regulate the level of hydrolysis such that the final product, whether in powder form or extrudate form or granular form is free of defects such as fines, fractures on the surface or internally etc. Similarly, resin processing is a technology in which it is imperative to control the levels of moisture present in the system so that molding of the resin products is rendered efficient and wastage of material is avoided. It is also imperative to control moisture levels during paper manufacture such that the final product has less than 3% moisture present.
As can be seen, different industries have different product moisture requirements, and in some cases optimal quality which requires the presence of ppm levels of moisture is not attainable without significant cost-increases in the manufacturing process and expenditure of high levels of energy.
Moisture content determination is generally done by two methods—direct and indirect. The former, viz. direct methods involve measurement of weight loss of the product using methods such as oven drying or chemical titration using Karl Fischer reagents. Direct methods generally provide a reasonably high level of accuracy, however, suffer from requiring off-line treatment. Thus, such methods are not suitable in industries where the level of automation is higher, and in fact even in other industrial methods suffer from the disadvantage of being energy inefficient and expensive.
Indirect methods of moisture determination involve the use of techniques such as electromagnetic wave measurement, the use of nuclear, dielectric or infra red sensors etc. While these methods provide contactless measurement, the applicants are unaware of any method by which sampling and averaging over the entire material volume has been achieved with significant degrees of accuracy, and also enabling control in real time. Such methods are known to provide for measurement online. However, the aspect of measurement and control proceeding concurrently and online or in-line has not as yet been explored.
Historically, methods of moisture level determination in manufacturing processes have involved off-line measurement. This has entailed stopping a process stream, removing a sample of the product stream, and determining the level of moisture therein, using equipment such as the Karl-Fischer reagent. Dependent on the readings obtained in the detection, the process parameters are then reset to ensure that the product stream achieves the required level of moisture presence or absence. As is evident, such processes suffer from a significant disadvantage in that they are not energy efficient, entail work stoppage and restart, and the levels of accuracy are not high.
For example, it was known that maintaining moisture content of organic resin materials at a constant level is important to ensure good quality of the resin products. Inappropriate levels of moisture content in resin materials being supplied to a moulding machine have resulted presence of such defects such as voids and silver lines in the product. It was conventionally known to dry such resin materials using a hopper dryer prior to supply of the materials to the moulding machine. This involved setting a fixed heating temperature and heating time for the hopper drier based on an estimation of the moisture content of the resin materials. The products are sampled and subjected to a titration analysis using a Karl Fischer reagent. While such methods do result in a reasonable degree of accuracy, they suffer from the attendant disadvantages of requiring off-line moisture measurement in a laboratory or by trained professionals, and are also not repeatable. As is evident, the stop-restart of the process is in itself a significant disadvantage in terms of energy inefficiency.
It is recognized that moisture content of materials is important in even food and agrochemical research and industrial applications. Generally, the drying techniques used in these industries involve oven drying based on drying samples under specific conditions of temperature and time depending on the material. However, such techniques are time consuming and are energy intensive. Additionally, the accuracy of such techniques is suspect due to differentials between the sample being tested and the mass distribution of the material in the product stream. Attempts have been made in such applications to utilize electromagnetic wave interaction for continuous measurement. While this method has one advantage of being non-contact, it is liable to fluctuations due to the variations in bulk density of the product stream. This requires that density also be continuously monitored separately, leading to enhanced energy and cost consumption, apart from making the measuring system extremely complex.
There are several prior arts in the field of moisture measurement or determination, including both online and offline measurements. Some of these are discussed below.
The requirement of moisture determination and control (emphasis added) in various industries will be discussed below with reference also to the art that is known in such industries, their advantages and their limitations.
In the field of ceramic forming materials, the use of extrusion processes is common. One type of an extrusion process that is used in this industry involves use of a ceramic-forming material that forms a plastic mix or “batch material”. This is extruded through a die orifice to form a shaped article. Ceramic honeycomb-shaped articles having a multitude of cells or passages separated by thin walls running parallel to the longitudinal axis of the structure have been formed through extrusion and used as filters for a variety of applications, including particulate filters for combustion engines. This process requires control of a number of parameters so that the desired article maintains its post-extrusion form. The parameters include, for example, the particular composition of the mix that makes up the batch material, and the moisture content of extruded logs that can subsequently be dried and fired to form a ceramic article. A batch material having insufficient moisture will not extrude properly and could lead to the formation of cracks, including invisible microcracks, in the final article. On the other hand, an organic batch material having too much moisture will also not extrude properly and could lead to deformation of the extruded article.
US Patent Publication 2010/300183 discloses an in-line method of measuring the moisture content of ceramic material within an extrusion system used to form ceramic articles. The method of this disclosure comprises arranging, at least one radio-frequency (RF) sensor system having an RF antenna relative to the extrusion system, and generating through the RF antenna an RF field that resides substantially entirely within the ceramic material; in response to the RF field interacting with the ceramic material, generating in the RF sensor system a signal SM representative of a raw moisture-content measurement of the ceramic material; generating calibration data by performing RF moisture-content measurements on samples of the ceramic material having different known moisture contents; and establishing a calibrated moisture-content measurement using the raw moisture-content signal SM and the calibration data. The method of this disclosure and the subsequent development thereof in US 2011/0006461 both rely on removal of a sample and testing thereof. Therefore, effectively, while the measurement levels are deemed significantly accurate, the control mechanism is still off-line. Therefore, the method would involve stoppage if alterations are to be done in the moisture levels. In addition, product could be made off-specification for a time until the process is adjusted.
U.S. Pat. No. 5,377,428 relates to papermaking drying processes and apparatus for producing paper with low moisture content. While this document states that it also relates to a control mechanism, the mechanism is specific to the paper manufacturing industry and is dedicated to measurement to 3% levels of moisture. The system of this disclosure comprises cross-direction drying means controllable to modify the temperature across the web, temperature detection means for determining the cross-direction temperature profile of the web, and modulation means for controlling the cross-direction drying means in response to variations in the temperature profile to produce an optimally uniform cross-direction temperature profile. The temperature detection means includes a high temperature detection means positioned where at least a portion of the web can have a temperature above the boiling point of water and, optionally, a low temperature detection means located where an entire cross-direction strip of the web will be at a temperature below the boiling point of water. Drying rate prediction means are further included to predict the drying rate of the web as a function of observed temperature in locations proximate to the low temperature detection means. The modulation means is responsive to signals from both the high and low temperature detection means to produce a substantially uniform cross-direction web temperature profile near the high temperature detection means. The drying control method of the present invention produces a substantially flat, uniform cross-directional profile by detecting the cross-direction web temperature, monitoring the cross-direction temperature profile as the web is dried, and controlling the rate of drying the web to insure that the web temperature is maintained at a substantially uniform optimum temperature and flat cross-directional profile. As can be seen, this system is limited in its applicability to paper manufacture and is entirely dependent on temperature measurement to enable control.
U.S. Pat. No. 6,439,027 discloses a method for gas moisture measurement, and more particularly, to particulate mass measurement instruments operable to measure the moisture content of effluent gas for real-time adjustment of isokinetic sampling during measurement of the mass of particulate matter flowing in a stack or other exhaust conduit. While the method appears to comprise moisture content measurement of the effluent gas in real time; obtaining an isokinetic sample of a portion of the effluent gas based on real-time moisture content measurement of the effluent gas, including a step of determining a proportion of water vapor by volume of the effluent gas, the actual measurement method appears to be offline is based on an isokinetic measurement.
U.S. Pat. No. 7,330,034 discloses a method and a system for moisture measurement in cotton bales. The process of this disclosure comprises measuring the moisture content and the mass-moisture content of materials without requiring air reference or calibration sequence. A microwave signal is split into a reference and a transmission signal, and the reference signal is applied directly to the phase detector, whereas the transmission signal is first transmitted through the sample before being presented to the other side of the phase detector. This measurement provides a phase-constant measurement that is due to the dielectric characteristics of the material under test. The system measures the material's phase-constant across a band of frequencies. The slope of the phase-constant versus frequency is then utilized to predict the density of the material which is then combined with the corrected phase-constant measurement to calculate the moisture content of the material. This disclosure is limited to determination of moisture content and does not provide on-line solutions for moisture content control.
U.S. Pat. No. 6,691,563 discloses what apparently is an invention to provide a method and apparatus for determining moisture content in any particulate or granular material at any radio-frequency and temperature without knowledge of bulk density from a single moisture calibration equation. However, the method of this disclosure requires the measurement at at least two points, and is moreover limited to measurement of dielectric constants. In addition, it is not very clear whether the method truly provides for absolutely on-line and continuous control. The method does appear to provide for measurement of moisture content, but the control algorithm would appear to have a significant time lag due to its complexity, and the need for multiple measurements.
The food processing industry is subject to stringent norms on the level of moisture content that is permitted in any given product. It is desirable in this area of technology to be able to guide the composition of food materials as to moisture content in order that tight control of processing can be maintained to assure efficient operation and efficient use of resources. The moisture content of liquid, semi-solid, or powdered food material has hitherto been determined by a variety of methods. For example, it is known to use a gravimetric method using either a vacuum or a microwave oven. Both variations are inconvenient to use when regular measurement of the moisture content of, e.g., hundreds of samples is required. The vacuum oven variation also requires undesirably long measurement times for on-line food processing. Other methods include measuring reflectance or absorption or some other parameter as an indirect indication of moisture content. However, all prior techniques have been found to lack at least one of several requirements for conveniently and meaningfully measuring moisture content of food materials. Such requirements include, illustratively, high temperature tolerance, short time to make a moisture content measurement of a sample, ease of calibration, and substantially no contamination between sensor and food system.
U.S. Pat. No. 5,257,532 discloses a method where moisture content measurements are alleviated by measuring the temperature-difference versus logarithm-of-heating-time response of a sample of a predetermined material during a predetermined heating time interval, determining the slope of that response at a portion thereof of approximately maximum slope, determining a temperature parameter related to the temperature at the start of heat application, and calculating the moisture content of the sample as a function of that slope and that temperature parameter. However this method also appears to require off-line testing of a sample, or at the very least involves a time lag between measurement, and then introduction of the control means, which may be not of significant use in increasingly automated industrial processes.
Apart from the above, there are several prior art disclosures known for moisture determination in different fields of technology such as soil testing, mining, agro-chemicals, polymer processing etc. However, the prior art known to applicants appear to focus on off-line testing followed by moisture content control, or alternatively have serious shortfalls in terms of moisture content levels that can be measured and controlled, as well in the time lags between measurement and control. The focus in the art appears to have been on off-line measurement strategies due to the relatively high levels of accuracy required, and also appear to have been industry specific. Indeed, applicants are not aware of a single measurement and control methodology which is universally applicable irrespective of the industry, and which provides measurement levels at the ppm range and provides for simultaneous control of moisture content.